Source location of the 26 sec microseism from cross-correlations of ambient seismic noise

Very strong, narrow band Rayleigh wave arrivals peaking near 26 sec are observed in cross- correlations of long time sequences of ambient noise records on US, European, and African stations with apparent arrival times that are typically smaller than expected for propagation along the great- circle linking the stations. Use of these apparent travel times to locate the source of narrow band arrivals shows that the waves originate off the west African coast in the Gulf of Guinea and propagate with an average speed of about 3.5 km/ s; i. e., close to the expected group speed of Rayleigh waves at this period. Using data from different months demonstrate that the source location is temporally stable with amplitudes that maximize during the southern hemisphere winter. A microseism with similar features but with a slightly broader spectral peak is observed at stations in East Asia and in the western Pacific, and apparently originates in the North Fiji Basin. The nearly antipodal location of the two source regions leads to the hypothesis that the 26 sec signals are excited by a single source located in the Gulf of Guinea. The Pacific inter- station cross-correlations may be for major- arc propagation.